Method of making diphosphoric esters



Patented Mar. 28, 1950 METHOD OF MAKING DIPHOSPHOBIC ESTERS Leonard W. Harris, Richmond, CaliL, asaignor to California Spray-Chemical Corporation, Richmond, Calm, a corporation of Delaware No Drawing. Application June 14, 1947, Serial No. 754,773

8 Claims. (Cl. 260461) This invention relates to a. method of making dlphosphoric esters having valuable properties, especially for use in controlling insect and other sts. I have discovered that by contacting trialkyl phosphate with chlorine under the conditions hereinafter described I am able to obtain a product which is very toxic to many animals, rodents and insects, and yet which can be applied in lethal concentration without injury to plant tissue or to fabrics infested with rodent or insect pests. Moreover, the said product is very unstable when exposed in dispersed form to the atmosphere, forming decomposition products which are non-toxic.

The product of the method of my invention is a complex mixture of phosphate, but owes its toxic property largely to a certain type of phosphoric ester which it contains in substantial amount. This toxic compound can be separated from the reaction product in substantially pure form by distillation under extremely high vacuum, as in a suitable type of molecular still. It is a tera-alkyl ester of a diphosphoric acid, although its specific formula has not been proved conclusively. Basically, the reaction leading to its formation involves the reaction of one mol of chlorine with two mols of trialkyl phosphate to give one mol of tetra-alkyl diphosphate and two mols of alkyl chloride. Other reactions occur, however, and the liquid product contains, in addition to the diphosphate, phosphorus compounds of undetermined structure. In practice the liquid product also usually contains some unreacted trialkyl phosphate. The gaseous alkyl chloride evolved separates readily and may be allowed to escape to the atmosphere or recovered by refrigeration.

The aforesaid tetra ester isolated by molecular distillation has extremely high insecticidal power, whereas the undistilled residue possesses substantially none. It reacts with potassium iodide to liberate iodine, and it decolorizes potassium permanganate solution. After a short induction period it decomposes rapidly at 150 0., even under very high vacuum, and slowly at temperatures as low as 100 C.

While the hereindescribed process may be carried out primarily for the production of material to be used as th toxic ingredient of insecticides and. as stated, the toxicity resides for the most part in the tetra alkyl ester of diphosphoric acid contained in the reaction product, it is not necessary that the ester be separated for such use. The entire product may be so used (although I prefer first to remove from it any dissolved and entrained chlorine it may contain). By the method of my invention I am able to manufacture a product having a high insecticidal potency, that is, containing a high percentage of the tetra ester described above, by a practical, economical continuous operation.

Various means may be used for contacting the chlorine with the liquid trialkyl phosphate. For best results such means should provide for intimate contact in a reasonably brief time and should be such that time and temperature can be controlled. I prefer, especially for continuous operation, a vertical column into the upper portion of which trialkyl phosphate is continuously introduced and into the lower portion of which chlorine gas is continuously introduced. The reactants thus flow countercurrently. Intimate mixing is provided by suitable packing or bailled construction within the column. I prefer for this purpose to use a type of packing which minimizes 7 liquid hold up within the column. For example,

the column may contain hollow rings of glass, ceramic or other unreactive material placed in a random manner. I provide means for establishing and controlling the reaction temperature, either by controlling the temperature of the materials entering the reaction zone or by controlling the temperature within the zone itself, or by a combination of these steps. For example, when a countercurrent column is used, it may be equipped with a thermal jacket, with internal coils, with electrical heating means, or with any similar device whereby the temperature of the reacting materials may be maintained at the desired point.

I also make provision for the removal of gas from the reaction zone. When the reaction zone is in the form of a countercurrent column, a gas outlet is provided in the upper portion, preferably above tlre point at which trialkyl phosphate is introduced. The gas so released consists for the most part of alkyl chloride which is one of the reaction products. It may also contain varying quantities of unreacted chlorine and small quantities of other reaction products.

The liquid reaction product which collects in the lower portion of the column usually contains some entrained or dissolved chlorine. When the product is to be used for insecticidal compounding I find it desirable to remove chlorine and any other gaseous material which may be associated with it. A convenient method for so doing consists in passing the liquid product through a vaporizing zone held at a moderately sub-atmospheric pressure.

I have found that the above-described reaction may be carried out within the temperature range of 110 to 190 C. I prefer for best results to maintain the reaction temperature within the range 130 to 160 C. In speaking of the reaction temperature I refer to the maximum temperature within the reaction zone. I find that this maximum temperature, in the case of a vertical countercurrent column, occurs at a point not far removed from the point of chlorine introduction, 1. e., in the lower portion of the column. The course of the reaction is influenced by the time of contact as well as by the temperature. If the apparatus and conditions of its operation are such as to afford a relatively long time of contact, the maximum temperature which can be tolerated and which is conducive to maximum yield of desired product, is lower than in cases where the contact time is short. I prefer to carry out the above-described reaction under conditions which will provide a time of reaction of from 2 to 5 minutes, and when so operating I have found that a maximum reaction temperature of about 150 C. gives good results. If, however, a contact time of to minutes exists, I would use a lower temperature of the order of 110 to 120 C. p

The weight ratio of chlorine and trialkyl phosphate needed to satisfy the desired reaction, i. e.,

the synthesis of tetra alkyl diphosphoric ester, is one mole of chlorine to two moles of trialkyl phosphate. It is not essential, however, that the materials be added in these proportions in order to obtain a reaction product having the properties described. It may, in view of the construction of the apparatus and the conditions of its use, be desirable in some cases to use more chlorine than stated above in order to secure adequate contacting with all of the trialkyl phosphate. On the other hand, if the apparatus and its operation are such that the time-temperature factor applied to the reaction is high, there is a tendency for secondary reaction of chlorine with the desired tetra ester. In such circumstances I minimize this tendency by reducing the relative amount of chlorine present in the reaction zone, using less than one mole of chlorine per two moles of trialkyl phosphate. This variation of weight ratios of the reactants and the selection of an optimum ratio is of importance only insofar as it relates to the yield of the desired diphosphoric ester. It is not determinative of the synthesis of such ester.

The following is an example of a practical embodiment of the method of my invention. The apparatus consisted essentially of a vertical reaction column having a length approximately ten times its diameter. This column was provided at the top with a gas outlet line and, near the top but below the outlet line it was provided with a reagent inlet. It was provided at the bottom with a product outlet line and, near the bottom but elevated above the outlet line, with a chlorine inlet. The column was equipped with an annular electrically heated jacket. The column was packed substantially throughout its length with glass rings placed in a random manner. Conventional means were provided for indicating temperatures at the inlet and outlet of the jacket and at several points within the column. Likewise, conventional means for indicating and regulating the flow of materials entering the column and the jacket were provided. Y

Preheated liquid triethyl phosphate (C2H5) aPOr, was introduced through the upper inlet line simultaneously with the introduction of chlorine gas through the lower inlet line. The jacket was maintained at a temperature such that the maximum temperature within the column was held steady at about 140 C. Gas was released from the top outlet at atmospheric pressure, and liquid product was withdrawn from the bottom outlet at such a rate as to maintain a low liquid level within the column. The liquid withdrawn from the column was passed through the vessel maintained at about one-half atmospheric pressure. This low pressure vessel was constructed so as to permit the incoming liquid to fall over baflies and so facilitate the release of dissolved chlorine. Following this dechlorination, the liquid product was run to storage. The rates of chlorine and triethyl phosphate were adjusted and controlled so as to maintain a weight ratio of chlorine to phosphate of about 70 to 364 (i. e., about 1 mole of chlorine per 2 moles of triethyl phosphate).

The gas released from the top of the reaction column consisted largely of ethyl chloride plus a small amount of chlorine. The liquid product withdrawn from the base of the column was passed through the vessel on which a moderate vacuum of about 2" of mercury was maintained.

Passage through this vessel was so conducted as to afford turbulent flow in a shallow stream, thereby permitting dissolved chlorine to be released and evacuated; The liquid product thus dechlorinated was colorless to light amber. It had a density of 1.18 to 1.25. It was soluble in chloroform, ketones and alcohol, but insoluble in petroleum distillates. It dissolved in water with decomposition. It corroded brass and mild steel actively, but reacted comparatively slightly at atmospheric temperature with stainless steel or aluminum. By subjecting this reaction product to a temperature of to C. at an absolute pressure of 1 to 2 microns Hg in a falling film type molecular still I obtained a distillate yield of 40% of the charge, consisting of tetra ethyl diphosphate, density 1.1983 at 20 C., refractive index 1.4190 at 22-5 0., molecular weght 304 (boiling point method) to 310 (freezing point method)theoretical 306. Comparative biological tests showed this substance to have twice the toxicity of the total reaction product. The undistilled residue from this molecular distillation was a liquid having an average molecular weight much higher than 306 and possessed substantially no insecticidal power. Hence biological testing of the total reaction product carried out by carefully controlled standardized procedures furnishes a good measure of the amount of tetra ethyl diphosphate contained therein.

When the process of my invention is carried out primarily for the production of material to be used in compounding insecticides, I prefer to use triethyl hosphate and chlorine, thus synthesizing a product containing a substantial amount of the tetra ethyl ester described above. Valuable insecticidal products can be made by the herein described reaction however, when other alkyl radicals are employed. I have found that such valuable insecticidal products result from the use of trialkyl phosphates in which the alkyl radical contains 1, 2 or 3 carbon atoms, that is, trimethyl-, triethyl, tripropyl, or triisopropyl phosphate. The toxic principle contained in the reaction product is therefore tetramethyl-, tetraethyl, tetrapropyl or tetraisopropyl diphosphate as the case may be. The use of trialkyl phosphates in which the alkyl radical contains 4 or more carbon atoms yields products which are inferior for insecticidal use, but have value for other uses such as use as plasticizers.

Alkyl chloride is a valuable product of the reaction described herein. It may be recovered by refrigeration or by absorption in alcohol. It is particularly desirable when carrying out this process with the ethyl ester to recover the ethyl chloride produced by the reaction.

I claim:

1. A process for the preparation of tetra-alkyl esters of a diphosphoric acid which comprises heating to a temperature of 110-'190 C. a mixture consisting essentially of a trialkyl phosphate and elemental chlorine in intimate admixture.

A process for the preparation of tetra-alkyl esters of a diphosphoric acid which comprises heating to a temperature of 130-160 C. a mixture consisting essentially of a trialkyl phosphate and elemental chlorine in intimate admixture.

3. A process for the preparation of tetra-alkyl esters of a diphosphoric acid which comprises heating to a temperature of 110-190" a mixture consistin essentially of a trialkyl phosphate and elemental chlorine in intimate admixture, and limiting the time of contact within such temperature range to not more than minutes.

4. A process for the preparation of tetra-alkyl esters of a diphosphoric acid which comprises heating to a temperature of 110-190 C. a mixture consisting essentially of a trialkyl phosphate in which the alkyl radicals contain from 1 to 3 carbon atoms and elemental chlorine in intimate admixture.

5; A process for the preparation of a tetraalkyl ester of diphosphoric acid which comprises heating to a temperature of 11-0-190 C. a mixture consisting essentially of a trialkyl phosphate and elemental chlorine in intimate admixture, separately removing from the reaction zone the gaseous and liquid reaction products and passing said liquid products through a subatmospheric pressure zone.

6. A process for the preparation of tetraethyl esters of a diphosphoric acid which comprises heating to a temperature of -190 C. a mixture consisting essentially-of triethyl phosphate and elemental chlorine in intimate admixture.

7. A process for the preparation of tetraethyl esters of a diphosphoric acid which comprises heating to a temperature of -160 C. a mixture consisting essentially of triethyl phosphate and elemental chlorine in intimate admixture.

8. A process for the preparation of tetraethyl esters of a diphosphoric acid which comprises heating to a temperature of 110-190" C. a mixture consisting essentially of triethyl phosphate and elemental chlorine in intimate admixture, and limiting the time of contact within such temperature range to not more than 15 minutes.

LEONARD W. HARRIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,411 Romieux et a1. June 15, 1937 1,763,852 Johnson June 17, 1930 2,060,815 MacAfee Nov. 17, 1936 2,063,629 Salzberg et a1 Dec. 8, 1936 2,343,831 Osborne Mar. 7, 1944 2,370,786 Fox Mar. 6. 1945 2,373,670 Engelke Apr. 17, 1945 OTHER REFERENCES Gerrard: "Jour. Chem. Soc. (London) vol. 1940. page 1468. 

1. A PROCESS FOR THE PREPARATION OF TETRA-ALKYL ESTERS OF A DIPHOSPHORIC ACID WHICH COMPRISES HEATING TO A TEMPERATURE OF 110-190*C. A MIXTURE CONSISTING ESSENTIALLY OF A TRIALKYL PHOSPHATE AND ELEMENTAL CHLORINE IN INTIMATE ADMIXTURE. 